FIG. 1 illustrates a conventional instrumentation amplifier 102 that includes buffered operational amplifiers OP1 and OP2 (also referred to as input buffers), and a differential operational amplifier OP3. The input buffers OP1 and OP2 provide a high input impedance, and eliminate the need for input impedance matching, thereby making the instrumentation amplifier 102 very useful for measurement and test equipment. For example, the instrumentation amplifier 102 is very useful for measuring the output of a sensor, such as, but not limited to, a strain gauge, photo detector, thermistor, thermocouple, temperature sensor, level sensor, current sensor, biometric sensor and Hall effect sensor. More generally, an instrumentation amplifier is useful for amplifying a relatively small differential signal that is superimposed on a relatively large common mode signal (e.g., a DC signal). This is because an instrumentation amplifier amplifies the difference between its two inputs (V1 and V2) while rejecting the signal that is common to the two inputs, to thereby produce its output (Vout).
Each operational amplifier of the instrumentation amplifier 102 can be implemented as a multi-path amplifier (sometimes referred to as a “feed-forward” amplifier), that includes separate low and high frequency feed-forward paths. Exemplary multi-path amplifiers 212B and 212C are shown in FIGS. 2B and 2C, respectively. Referring first to FIG. 2A, the multi-path amplifier 212A is generally shown as having a low frequency path including at least transconductance stages Gms, Gmi and Gmout, and a high frequency path including transconductance stage Gmf. Each transconductor stage (e.g., Gms, Gmi, Gmout and Gmf) can also be referred to simply as a transconductor. The capacitors Ccs and Ccf provide for parallel integration paths for low and high frequency.
Each of the transconductance stages in FIG. 2 is assumed to have a very high (but finite) output impedance. The unity gain frequency for the low frequency path is proportional to Gms/Ccs. The unity gain frequency for the high frequency path is proportional to Gmf/Ccf. Setting Gms/Ccs=Gmf/Ccf allows for a clean 20 dB/decade roll-off for the overall open loop transfer, which is a very desirable characteristic for the amplifier. FIG. 2B illustrates a specific implementation of a multi-path amplifier 212B. The multi-path amplifier 212B can be referred to as “3-stage multi-path” amplifier, because there are three transconductance stages in the low frequency path, and there are multiple paths between the input and output of the amplifier 212B. FIG. 2C illustrates an exemplary 4-stage multi-path amplifier 212C. Depending on implementation, additional transconductance stages can be added, as can choppers, filters, etc.
In multi-path amplifiers, such as those in FIGS. 2A, 2B and 2C (but not limited thereto), the transconductances Gms and Gmf (of the differential input transconductance stages of the low and high frequency paths) are conventionally set such that they are equal in order to give a flat voltage noise response over frequency up to the bandwidth of the amplifier. Here it is assumed that the low frequency (e.g. 1/f) noise is negligible, this can be achieved using circuit techniques such as chopping. Stated another way, Gms=Gmf. Further, the capacitors Ccs and Ccf are conventionally set such that they are equal, i.e., Ccs=Ccf. This also results in Gms/Ccs=Gmf/Ccf.
FIG. 3 illustrates the flat noise spectral density response of a conventional instrumentation amplifier (e.g., 102 in FIG. 1) that is implemented using multi-path amplifiers (e.g., 212A, 212B or 212C in FIGS. 2A, 2B and 2C) having the conventional transconductance and capacitor values just described above. Here it is assumed that the low frequency (e.g. 1/f) noise is negligible, which can be achieved using circuit techniques such as chopping. Additionally, an infinite bandwidth is assumed, and only the ideal white noise profile of the amplifier is represented.
While the flat frequency response shown in FIG. 3 is sometimes desirable, other responses may be acceptable or desirable, depending on the application. Further, depending on the application and/or the larger circuit in which an instrumentation amplifier is incorporated, it may be desirable to reduce the current and power drawn by an instrumentation amplifier. For example, where an instrumentation amplifier is incorporated into a portable device that draws current and power from a battery, it would be beneficial to reduce the current and power drawn from the battery, to thereby increase the time between battery charges or replacement.